The Time Of Arrival (TOA) of radio signals is used by many applications running on wireless networks for synchronizing the network clocks, for measuring the distance between two devices, and/or for computing device positions. The accuracy of clock synchronization and of position services (i.e. Geographic, Geocentric or Relative positioning) depends on the accuracy of the measurement of the Time Of Arrival. From TOA can be determined the Time Of Flight (TOF), the Time Difference Of Arrival (TDOA), the clock shift and other time related entities that can be used in specific algorithms for computing the position of an object in relation with others or for synchronizing clocks in a wireless network.
One method for synchronizing clocks of wireless network nodes or for measuring distances between wireless devices includes the transmission of a radio impulse. The receiver identifies the moment when the impulse is received and records the clock value. The precision of the method depends on the length of the impulse and the clock resolution. For providing accurate precision, the impulse has to be very short. This has two undesired consequences: (a) because of its short length (preferably less than one nanosecond) the signal has very little energy and cannot propagate very far; and (b) the signal has many harmonics of almost equal energy spread over a large spectrum. Due to these undesired consequences, the impulse method has little practical application.
Traditionally, in wireless digital networks, the TOA is measured using the impulse response method that results from autocorrelation. According with this method, a pseudo-random sequence of binary modulated signals is transmitted. The received signal is correlated with the same pseudo-random sequence. In time, the autocorrelation function provides an almost flat response except for at the moment when the received sequence matches the reference sequence of pseudo-random binary symbols. The shape of the response of the autocorrelation function is similar to an impulse, although the signal from which it is generated has a relative narrow band and does not require large amount of energy.
FIG. 1 illustrates a typical impulse response for a pseudo-random sequence using binary phase-shift keying (BPSK) modulation. The receiver uses the peak of the impulse response for synchronizing to the received signal. The precision of the autocorrelation method is in inverse relation with the duration of transmission of one binary symbol, therefore with the width of the radio channel. For example, using a twenty (20) Megahertz (MHz) channel, a fifty (50) nanosecond precision of TOA is achieved with this method, while using a one hundred (100) MHz width channel, the achieved precision is ten (10) nanosecond.
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